1. Field of the Invention
The present invention relates to a tape cassette having a lock member for locking a tape in a reel.
2. Detailed Description of the Related Art
A tape cassette such as a magnetic tape cassette has a reel for winding a tape thereon. The reel is constituted by a reel drum on which the tape is wound, and a flange. As for the flange, there are a case where two flanges are provided on the upper and lower sides as in a VHS video tape cassette, another case where only one flange is provided on either one of the upper and lower sides as in a U-matic video tape cassette, and a further case where no flange is provided in any of the upper and lower sides as in an audio tape cassette. When a flange or flanges are provided, generally, the flange on one side (on the lower side in the case of a VHS video tape cassette) is formed integrally with a reel drum. According to the present invention, a reel drum includes that which has a flange formed integrally therewith. In the reel drum, a lock portion for locking an end of a tape is provided in the reel drum, and the tape is wound on the reel drum while the end of the tape is locked in the lock portion by a lock member. In the case of a magnetic tape, there are a case where the end of the tape is locked directly, and another case where a leader tape or a trailer tape is connected to the end of the magnetic tape and the end of the leader or trailer tape is locked in the reel drum.
Conventionally, the reel drum had been formed from polyacetal resin which was superior in abrasion resistance. However, polyacetal resin is large in density so that it makes the reel heavy. In addition, polyacetal resin cannot be bonded by welding directly with a flange formed from acrylonitrile-styrene copolymer or the like so that it makes the structure of parts or the process of manufacturing complicated. Further, there is a problem that formalin gas generated from polyacetal resin rusts a deposited tape or the like. Therefore, polyacetal resin has been replaced by polystyrene resin, a resin which is light in weight and which does not generate gas, such as resin shown in Examined Japanese Patent Publication No. Hei-5-68787, or the like.
However, when the lock member is formed from polystyrene resin, it is disadvantageous in that, for example, the lock member is easy to crack, enough locking force cannot be obtained, and the like. Accordingly, the lock member is still formed of polyacetal resin.
When a drum made from polystyrene resin and a lock member formed from polyacetal resin are combined, the density of polyacetal resin is so large that the deflection of a reel is increased at the time of high-speed rotations by a difference in density between polyacetal resin and polystyrene resin. Accordingly, disorder is generated in winding of a tape.
When a molded body is made from resin, there is a tendency that the size increases near a gate from which resin is injected into a mold, and the size decreases at a position away from the gate. Accordingly, when the molded body is bilateral symmetrical like the lock member according to the present invention, the dimensional accuracy of the product is obtained more easily if the gate is positioned at the center of the molded body. However, polyacetal resin is easier to crack in a weld portion (a line caused by impact by resins against each other in the mold) than any other resin, so that the gate cannot be positioned at the center of the lock member. Accordingly, the gate is shifted to the right or the left from the central line. When the position of the gate is shifted from the center, making the mold is more difficult than the case where the gate is formed at the center. Accordingly, it takes more time to make the mold, thereby resulting in an increase in cost. In the lock member, the dimensional accuracy on a difference in level between the drum surface and the upper rib surface of the lock member is particularly important. When the position of the gate is shifted from the center, the scattering of the size becomes larger than the case where the position of the gate is at the center, even if the mold is manufactured by spending much time and high cost as mentioned above.
Besides the above-mentioned problems, polyacetal resin generally is disadvantageous in that deposit is caused by gas on the mold surface during molding so that the quality of a molded product is deteriorated, and if things come to the worst, there may be a case where resin cannot flow, or a slide portion of the mold cannot slide so that the mold is broken. As a result, the life of the mold is shortened in comparison with any other resin.